1. Field of the Invention
The present invention relates to a photoelectric converting device, and particularly to a photoelectric converting device having pixels arranged in a two-dimensional array.
2. Description of the Prior Art
Two-dimensional photoelectric converting devices are used in various applications. A two-dimensional photoelectric converting device has pixels arranged in a matrix (two-dimensional array), and those pixels each include a photoelectric converting element (photosensitive element) such as a photodiode and a means for transferring the photoelectric charge generated in the light receiving element to an output signal line. Such photoelectric converting devices are roughly grouped into CCD-type and MOS-type devices. CCD-type devices achieve transfer of photoelectric charge while accumulating it in potential wells, and thus has the disadvantage of a narrow dynamic range. On the other hand, MOS-type devices directly read the charge accumulated in the pn junction capacitance of the photodiodes.
Now, how each pixel is configured in a conventional MOS-type photoelectric converting device will be described with reference to FIG. 47. As shown in this figure, a photodiode PD has its cathode connected to the gate of a MOS transistor T101 and to the source of a MOS transistor T102. The MOS transistor T101 has its source connected to the drain of a MOS transistor T103, and this MOS transistor T103 has its source connected to an output signal line VOUT. A direct-current voltage VPD is applied to the drain of the MOS transistor T101 and to the drain of the MOS transistor T102, and a direct-current voltage VPS is applied to the anode of the photodiode.
When light enters the photodiode PD, photoelectric charge is generated therein, and this electric charge is accumulated at the gate of the MOS transistor T101. Here, when a pulse signal φV is fed to the gate of the MOS transistor T103 to turn the MOS transistor T103 on, a current proportional to the charge accumulated at the gate of the MOS transistor T101 flows through the MOS transistors T101 and T103 to the signal output line. In this way, it is possible to read the output current that is proportional to the amount of incident light. After this signal has been read, the MOS transistor T103 is turned off and thereby the MOS transistor T102 is turned on so that the gate voltage of the MOS transistor T101 will be initialized.
As described above, in a conventional MOS-type photoelectric converting device, at each pixel, the photoelectric charge generated in the photodiode PD and then accumulated at the gate of the MOS transistor T101 is directly read out. This, however, leads to a narrow dynamic range and thus demands accurate control of the amount of exposure. Moreover, even if the amount of exposure is controlled accurately, the obtained image tends to suffer from flat blackness in dim portions thereof and saturation in bright portions thereof.
On the other hand, the assignee of the present invention has once proposed a photoelectric converting device including a photosensitive element that generates a photoelectric current in accordance with the amount of incident light, a MOS transistor to which the generated photoelectric current is fed, and a bias circuit that supplies a bias to the MOS transistor to bring it into a state in which a subthreshold current flows therethrough, wherein the photoelectric current is subjected to logarithmic compression conversion (refer to U.S. Pat. No. 4,973,833). This photoelectric converting device offers a wide dynamic range, but there is room for improvements of performance and S/N (signal-to-noise) ratio in low-light conditions (low-brightness conditions). Moreover, this photoelectric converting device also suffers from a comparatively large pixel size, because an integrator circuit having a capacitor needs to be incorporated in each pixel.